The borehole televiewer is a tool which provides a detailed sensing of the reflectance and topographic properties of a borehole surface, thereby giving a circumferential "picture" of the borehole wall. This imaging is accomplished through the use of a transmitter and receiver notably mounted in a logging tool, or alternatively a single transducer capable of both functions. The transducer or transmitter/receiver combination is continuously rotated within the wellbore, wherein the transmitter/transducer produces a series of pulses of acoustic energy. These pulses travel to the periphery of the borehole, where they impinge upon the borehole wall, and return as an echo signal to the receiver/transducer. This received echo pulse is then applied to an amplifier, the output of which is used to modulate a display of the intensity of the received signal. In addition to conducting the reflected energy signals to the display device, means for determining the tools orientation within the wellbore is provided which utilizes an orientation pulse related to a geographic direction. One such borehole televiewer is described in U.S. Pat. No. 3,369,626, which is incorporated by reference herein for all purposes.
As the transmitted pulse from the transmitter/transducer impinges upon the surface of the borehole wall, some energy is specularly reflected according to the laws of geometric optics, and some energy is scattered in all directions due to the rugosity of the borehole well surface. The amount of such scattered energy as compared to that specularly reflected is controlled by said rugosity on a scale comparable to or smaller than the wavelength of the incident ultrasonic energy.
The extreme sensitivity of the amplitude measurement of the reflected signal, to the angle of incidence at which the signal is reflected, severely restricts the area over which any data from a conventional borehole televiewer is obtained due to the limitations of the typical borehole televiewer design. Typical designs utilize a single transmitter/transducer which produces an ultrasonic beam having an angle of about 3.degree., with rapidly decreasing power levels to about 6.degree.. If the angle of incidence of the reflected signal to the borehole wall is not perpendicular, the angularly reflected signal will not be picked up by the receiver/transducer, being scattered instead within the wellbore. Attempts to measure these scattered signals have been suggested wherein means are provided for measuring the extent of variation or decay in the reflection signal from a maximum value for each position of the rotating transducer. This variation corresponds to the amplitude variation away from a perpendicular specular reflection path. A numerical value may then be assigned to define the sharpness or flatness of the curve defining such variation. However, such a method merely relates these non-specular signals to a measured specular signal, giving no unique information on the signal itself.
It is therefore an object of the present invention to provide an apparatus and method for making ultrasonic measurements of the energy scattering properties of the surface of a borehole surface in order to characterize its rugosity.
It is more particularly an object of the present invention to provide an improved means for providing an improved signal-to-noise ratio of reflected echo signals by measuring said signals individually as directly reflected signals.